This invention relates generally to cryogenic dry shippers, and more particularly concerns a closure for closing the top opening of such dry shippers.
Dry shippers are atmospheric pressure cryogenic dewars that hold a liquid cryogen in an absorbent material so that the cryogenic liquid does not come in contact with the contents of the dry shipper. Dry shippers are generally used to ship frozen biological samples by common carriers and are designed to keep the samples frozen until they arrive at their destination.
Because such dry shippers are shipped by common carriers, they may be shipped in all attitudes despite the "this side up" labels on the shipping cartons. When a dry shipper is placed on its side or inverted, the holding time, the time in which the sample remains frozen, is dramatically reduced. The reason for degradation in the dry shipper's performance is the design of the dewar closure. In a conventional dry shipper, the neck opening is closed with a cork. The cork is constructed of an insulating material such as styrofoam that blocks outside heat from entering the open neck tube. The cork is fitted to the neck tube so that there is an peripheral space around the cork that allows for escape of gas to alleviate pressure as the cryogenic liquid evaporates from the absorbent material. The styrofoam cork with the peripheral space works fine while the dry shipper is upright because cold gas rests in the bottom of the dewar and has no path out. When the dry shipper, however, is tipped on its side or inverted, the cold gas can now flow along the bottom of the pressure relief peripheral space between the neck tube and the cork and be replaced by warm gas flowing in the peripheral space along the top of the cork. This convection flow, resulting from the differences in density between the cold gas adjacent the bottom of the peripheral space and the warm gas adjacent the top of the peripheral space, rapidly warms the inside of the dewar and decreases its holding time.